The chemistry of milk varies strikingly across mammalian species, reflecting evolutionary adaptations to the nutritional needs of their offspring, environmental conditions, and physiological differences in the mammary gland. Human milk is uniquely tailored for the rapid brain development of infants, featuring high lactose content, moderate fat, lower protein, and an abundance of bioactive oligosaccharides that promote gut microbiota and immune function. In contrast, cow's milk—the global standard for dairy production—contains higher protein (especially casein) and a different fat profile rich in medium-chain saturated fatty acids. Buffalo milk stands out for its exceptionally high fat (7–10%) and protein content, giving it superior cheese-yielding properties, whereas goat milk is naturally lower in casein α-s1 fraction, making it more digestible for some individuals sensitive to cow's milk. Sheep milk is the richest in fat and protein among common dairy species, explaining its traditional use in high-yield cheeses such as Roquefort and Pecorino. Camel milk, adapted to arid environments, has lower lactose and fat but uniquely high levels of vitamin C, insulin-like proteins, and protective antimicrobial compounds, while donkey milk most closely resembles human milk in overall composition, with low fat, low casein, high lactose, and a whey protein profile rich in lysozyme and lactoferrin. These compositional differences—in macronutrients (lactose, fat, protein), mineral content, vitamin profiles, and bioactive minor components—profoundly influence not only the growth rate and immune development of the young of each species but also the technological, nutritional, and therapeutic applications of these milks in human diets worldwide.
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